FEATURE: Serve localized content in the site's default locale when user's language is unsupported (#36160)

The content localization feature we have (e.g. on meta) currently
assumes that everyone has set their user preferences language to
a language that we support.

If Eleni sets Greek as her Interface language, it is a non-supported
language. Posts written in Chinese or French will remain in Chinese or
French, as there is no Greek translation.

The commit shows Eleni localized posts in English (i.e. site default locale)
when a post is written in a different language. A site setting
`content_localization_use_default_locale_when_unsupported` is default
true and introduced.

app/models/concerns/localizable.rb

@@ -5,9 +5,13 @@ module Localizable
 
   included { has_many :localizations, class_name: "#{model_name}Localization", dependent: :destroy }
 
-  # Returns the localization for the given locale, or the best match if an exact match is not found.
+  # Returns the localization for (in order of priority):
+  # - the given locale,
+  # - or the best match if an exact match is not found
+  # - or the site default locale if `content_localization_use_default_locale_when_unsupported` enabled
+  #
   # The query used to find the localization is optimized for performance, and assumes
-  # that localizations are indexed by locale, and have been preloaded where necessary.
+  # that localizations are indexed by locale, and have been preloaded.
   # @return [Localization, nil] the localization object for the given locale, or nil if no match is found.
   def get_localization(locale = I18n.locale)
     locale_str = locale.to_s.sub("-", "_")
@@ -17,7 +21,14 @@ module Localizable
       return match
     end
 
-    localizations.find { |l| LocaleNormalizer.is_same?(l.locale, locale_str) }
+    if match = localizations.find { |l| LocaleNormalizer.is_same?(l.locale, locale_str) }
+      return match
+    end
+
+    if SiteSetting.content_localization_use_default_locale_when_unsupported
+      default_locale = SiteSetting.default_locale.to_s.sub("-", "_")
+      localizations.find { |l| LocaleNormalizer.is_same?(l.locale, default_locale) }
+    end
   end
 
   def in_user_locale?

config/locales/server.en.yml

@@ -2773,6 +2773,7 @@ en:
     content_localization_allowed_groups: "Groups allowed to update localized content. Requires 'content localization enabled'."
     content_localization_language_switcher: "Show a language switcher in the header, allowing visitors to switch between translated versions of Discourse and user-contributed content. Uses languages defined in 'content localization supported locales'"
     content_localization_crawler_param: "Serve localized content to web crawlers when 'content localization enabled' and 'set locale from param' is enabled. The list of supported locales is defined in 'content localization supported locales'."
+    content_localization_use_default_locale_when_unsupported: "Serve localized content in the site's default locale to users whose preferred language is not listed in 'content localization supported locales'."
     enable_upcoming_changes: "Enable upcoming changes"
     fake_upcoming_change: "This is a fake upcoming change for testing purposes. No need to translate this string."
 

config/site_settings.yml

@@ -1745,6 +1745,9 @@ content_localization:
   content_localization_crawler_param:
     default: false
     area: "localization"
+  content_localization_use_default_locale_when_unsupported:
+    default: true
+    area: "localization"
 
 email:
   email_time_window_mins:

spec/models/tag_spec.rb

@@ -432,7 +432,7 @@ RSpec.describe Tag do
       end
 
       it "returns nil when no localization exists" do
-        Fabricate(:tag_localization, tag:, locale: "en")
+        Fabricate(:tag_localization, tag:, locale: "el")
 
         expect(tag.get_localization("es")).to be_nil
       end

spec/serializers/basic_post_serializer_spec.rb

@@ -22,13 +22,27 @@ RSpec.describe BasicPostSerializer do
         expect(json[:cooked]).to eq(post.cooked)
       end
 
-      it "returns the localized cooked" do
-        SiteSetting.content_localization_enabled = true
-        Fabricate(:post_localization, post: post, cooked: "X", locale: "ja")
-        I18n.locale = "ja"
-        post.update!(locale: "en")
+      describe "localizations" do
+        it "returns the localized cooked" do
+          SiteSetting.content_localization_enabled = true
+          Fabricate(:post_localization, post: post, cooked: "X", locale: "ja")
+          I18n.locale = "ja"
+          post.update!(locale: "en")
 
-        expect(json[:cooked]).to eq("X")
+          expect(json[:cooked]).to eq("X")
+        end
+
+        it "returns the site default locale cooked when no exact match found and `content_localization_use_default_locale_when_unsupported` is true" do
+          SiteSetting.content_localization_enabled = true
+          SiteSetting.content_localization_use_default_locale_when_unsupported = true
+          SiteSetting.default_locale = "el"
+
+          Fabricate(:post_localization, post:, cooked: "site default cooked", locale: "el")
+          I18n.locale = "ja"
+          post.update!(locale: "en")
+
+          expect(json[:cooked]).to eq("site default cooked")
+        end
       end
     end
   end

spec/serializers/basic_topic_serializer_spec.rb

@@ -10,15 +10,36 @@ describe BasicTopicSerializer do
       expect(json[:basic_topic][:fancy_title]).to eq(topic.title)
     end
 
-    it "returns the fancy title with a modifier" do
-      SiteSetting.content_localization_enabled = true
-      Fabricate(:topic_localization, topic:, fancy_title: "X", locale: "ja")
-      I18n.locale = "ja"
-      topic.update!(locale: "en")
+    describe "localizations" do
+      it "returns the fancy title with a modifier" do
+        SiteSetting.content_localization_enabled = true
+        Fabricate(:topic_localization, topic:, fancy_title: "X", locale: "ja")
+        I18n.locale = "ja"
+        topic.update!(locale: "en")
 
-      json = BasicTopicSerializer.new(topic).as_json
+        json = BasicTopicSerializer.new(topic).as_json
 
-      expect(json[:basic_topic][:fancy_title]).to eq("X")
+        expect(json[:basic_topic][:fancy_title]).to eq("X")
+      end
+
+      it "returns the site default locale fancy title when no exact match found and `content_localization_use_default_locale_when_unsupported` is true" do
+        SiteSetting.content_localization_enabled = true
+        SiteSetting.content_localization_use_default_locale_when_unsupported = true
+        SiteSetting.default_locale = "el"
+
+        Fabricate(
+          :topic_localization,
+          topic:,
+          fancy_title: "site default fancy title",
+          locale: "el",
+        )
+        I18n.locale = "ja"
+        topic.update!(locale: "en")
+
+        json = BasicTopicSerializer.new(topic).as_json
+
+        expect(json[:basic_topic][:fancy_title]).to eq("site default fancy title")
+      end
     end
   end
 end

spec/system/content_localization_spec.rb

@@ -29,7 +29,11 @@ describe "Content Localization" do
     )
   end
   fab!(:post_3) { Fabricate(:post, topic:, locale: "ja", raw: "将とは、智・信・仁・勇・厳なり。") }
+  fab!(:topic_ja_localization) do
+    Fabricate(:topic_localization, topic:, locale: "ja", fancy_title: "孫子兵法からの人生戦略")
+  end
 
+  # page objects
   let(:topic_page) { PageObjects::Pages::Topic.new }
   let(:topic_list) { PageObjects::Components::TopicList.new }
   let(:composer) { PageObjects::Components::Composer.new }
@@ -38,10 +42,6 @@ describe "Content Localization" do
   let(:post_3_obj) { PageObjects::Components::Post.new(3) }
   let(:post_4_obj) { PageObjects::Components::Post.new(4) }
 
-  fab!(:topic_ja_localization) do
-    Fabricate(:topic_localization, topic:, locale: "ja", fancy_title: "孫子兵法からの人生戦略")
-  end
-
   def scroll_to_post(post_number)
     5.times do
       break if page.has_css?("#post_#{post_number} .cooked", visible: :all, wait: 0)
@@ -371,6 +371,82 @@ describe "Content Localization" do
         expect(page).to have_title(shady_topic.title)
       end
     end
+
+    context "for a Greek user in an English forum with Japanese users" do
+      fab!(:greek_user) { Fabricate(:user, locale: "el") }
+
+      fab!(:jap_post) { Fabricate(:post, locale: "ja", cooked: "皆さんは「ジョジョの奇妙な冒険」をご存知ですか？") }
+      fab!(:jap_topic) do
+        jap_post.topic.tap { |t| t.update(locale: "ja", fancy_title: "ジョジョの奇妙な冒険") }
+      end
+      fab!(:en_loc_jap_post) do
+        Fabricate(
+          :post_localization,
+          locale: "en",
+          post: jap_post,
+          cooked: "Do you know “JoJo’s Bizarre Adventure”?",
+        )
+      end
+      fab!(:en_loc_jap_topic) do
+        Fabricate(
+          :topic_localization,
+          locale: "en",
+          topic: jap_topic,
+          fancy_title: "JoJo's Bizarre Adventure",
+        )
+      end
+
+      before do
+        SiteSetting.default_locale = "en" # explicit
+        SiteSetting.content_localization_use_default_locale_when_unsupported = true
+      end
+
+      context "for a topic / post with no locale" do
+        it "shows content as-is" do
+          jap_post.update(locale: nil)
+          jap_topic.update(locale: nil)
+
+          sign_in(greek_user)
+
+          topic_page.visit_topic(jap_topic)
+          expect(topic_page).to have_topic_title(jap_topic.fancy_title)
+          expect(post_1_obj).to have_cooked_content(jap_post.cooked)
+
+          SiteSetting.content_localization_enabled = false
+
+          page.refresh
+          expect(topic_page).to have_topic_title(jap_topic.fancy_title)
+          expect(post_1_obj).to have_cooked_content(jap_post.cooked)
+        end
+      end
+
+      context "for a topic / post written in Site default language (en)" do
+        it "shows Site default language (en) translation to Greek user" do
+          sign_in(greek_user)
+
+          topic_page.visit_topic(jap_topic)
+          expect(topic_page).to have_topic_title(en_loc_jap_topic.fancy_title)
+          expect(post_1_obj).to have_cooked_content(en_loc_jap_post.cooked)
+
+          SiteSetting.content_localization_use_default_locale_when_unsupported = false
+
+          page.refresh
+          expect(topic_page).to have_topic_title(jap_topic.fancy_title)
+          expect(post_1_obj).to have_cooked_content(jap_post.cooked)
+        end
+      end
+
+      it "shows content as-is when no localization exists" do
+        en_loc_jap_topic.destroy
+        en_loc_jap_post.destroy
+
+        sign_in(greek_user)
+
+        topic_page.visit_topic(jap_topic)
+        expect(topic_page).to have_topic_title(jap_topic.fancy_title)
+        expect(post_1_obj).to have_cooked_content(jap_post.cooked)
+      end
+    end
   end
 
   context "for site settings" do

spec/system/page_objects/components/post.rb

@@ -28,8 +28,12 @@ module PageObjects
         find("#embedded-posts__bottom--#{@post_number} .load-more-replies").click
       end
 
+      def cooked_content
+        post.find(".contents > .cooked")
+      end
+
       def has_cooked_content?(value)
-        post.find(".contents > .cooked").has_content?(value)
+        cooked_content.has_content?(value)
       end
 
       def has_replies?(count: nil)